Magnetic blast fuses



Feb. 7, 1956 MAGNETIC BLAST FUSES Filed Oct. 2, 1953 United StatesPatent Ofiice 1 2,734,110 MAGNETIC BLAST FUSES Philip C. Jacobs, Jr.,Newtonville, Mass., assignor to The Chase Shawmut Company, a corporationof Massachusetts Application October 2, 1953, Serial No. 383,746 5Claims. (Cl. 200120) This invention refers to fuses and moreparticularly to fuses of the current-limiting type. l

The fuse links arranged in conventional current-l1m1ting fuses aresurrounded by a pulverulentarc-quenching filler which has a high heat orenergy absorbing capacity and is thus capable of absorbing theconsiderable amounts of energy which are fed into the fuse by thecircuit under interruption between the time of arc-initiation and thetime of arc-extinction. In current-limiting fuses of the so-calledsilver-sand type the high latent heat of fusion of the quartz sandsupplies the high energy absorbing capacity which is required forcurrent-limiting c1rcu1t interruption.

The interrupting process is initiated in any fuse by initial fusion andvaporization of the fuse link at a predetermined point, or predeterminedpoints, thereof. The resistance of the arc gap immediately uponformation thereof is very small, due to the small length which itinitially has. Current-limiting interruption of a circuit calls for avery rapid rate of rise of the arc voltage. The requisite rapid rate ofrise of the arc voltage is achieved in conventional current-limitingfuses by a design of their fuse links which is conducive to a rapidgrowth of the arc gap initially formed. There is, however, an inherentdraw-back in any fuse wherein gap growth and are elongation are mainly,or substantially, achieved by the burning action of the arc terminalsinitially formed on the fuse link. Considering at a given time in asubstantially linear are formed by the fusion of a fuse link, the pointof arc initiation being situated midway between the terminals of theare, it is apparent that are duration is longest at the point of arcinitiation and least immediately adjacent the terminals of the are onthe two spaced ends of the link between which the arc extends. The heatgenerated at any point of the arc is proportional to the duration of theare at the particular point. It follows therefrom that the heatgenerated by the arc is largest at the point of arc initiation and leastimmediately adjacent the terminals of the arc. The rate of dielectricrecovery of an arc gap at any given point thereof isinversely proportional to the duration of the are, or to the total are energyreleased at the particular point of the arc gap. It follows therefromthat the point of arc initiation is, or tends to be, a dielectricallyweak point in conventional, or prior art, current-limiting fuses.

It is, therefore, an object of this invention to provide acurrent-limiting fuse wherein the point of arc initiation tends to bedielectrically strong, i. e. does not tend to break down under theimpact of the recovery voltage.

Another object of this invention is to provide acurrentlimiting fusewherein the are is, or the incandescent products thereof are, blasted byelectromagnetic action away from the pointof arc initiation toward apoint or a region remote from the point of arc initiation.

Still another object is to provide a current-limiting fuse in which adynamic blast action rather than static cooling means is primarilyresponsible for the current-limiting and are extinguishing action.

A further object of the invention is to provide a currentlimiting fusewhich includes a structure in the nature of a miniaturized arc chute.

Still another object of the invention is to provide. fuses having animproved interrupting ability by virtue of simple, highly effectivemagnetic blow-out means.

It is also an object of the invention to generally im- 2,734,110Patented Feb. 7, 1956 2 prove fuses, and more particularlycurrent-limiting fuses, including reducing the bulk thereof.

For a better understanding of the invention reference may be had to theaccompanying drawing in which:

Fig. l is a top plan view of a fuse link;

Fig. 2 is a side elevation of the fuse link shown in Fig. 1;

Fig. 3 is a cross-section of a barrier and of a housing foraccommodating the fuse link shown in Figs. 1 and 2;

Fig. 4 is a perspective view of a complete fuse comprising the partsshown in Figs, 1 to 3, inclusive;

Fig. 5 is a vertical cross-section of the fuse structure shown in Fig.4;

Fig. 6 is a side elevation of the. current-carrying parts of anotherfuse embodying the present invention; and

Fig. 7 is a cross-section of a fuse, comprising the current-carryingparts shownin Fig. 6.

Referring now to Figs. 1 to 5, inclusive, reference numeral lindicatesgenerally a fuselink of the ribbon type preferably made of silver orcopper. Fuse link 1 is provided with a portion 3 of restrictedcross-section to initiate arcing at the occurrence of relatively largefault currents in an electric system to be protected. The restrictedcross-section portion 3 may consist of one or more current bridgesextending between the terminal portions of the link which may have acomparatively large cross-sectional area. In the structure shown in Fig.1 the terminal portionsof the link 1 are bridged by two current bridgesof which each may have, say, 4 of the cross-sectional. area of theterminal portions. As best shown in Fig. 2 fuse link 1 is bent to form acurrent loop with the apex portion thereof situated adjacent therestricted cross-section portion 3 to produce a magnetic fieldconcentration adjacent the restricted crosssection portion 3. Referencenumeral 2 has been ap.- plied to indicate the two sides of the U-loop'which are substantially parallel. When thefuse isbeing assembled abarrier 6 of heat resistant insulating material isinserted between theinner surfaces of the portions-2 of link 1, and barrier 6 and linkportions 2 are inserted into a hood 4 which .is also made of a heatresistant insulating material. Barrier or insulating spacer 6 and hoodor cover 4 may be made of a suitable ceramic material, e. g. a firedzircon ceramic. The, hood or; cover structure 4 has an orifice 5adjacent the restricted cross-section portion 3 of link 1 to permit theescape of products of arcing formed at the restricted cross-sectionportion 3 upon fusion thereof. Since in the embodiment shown in Figs. 1to 5, inclusive, the restricted cross-section portion 3 of link 1consists of two current bridges, a venting orifice 5 in hood 4 isassociated with each of the two bridges. The magnetic fieldconcentration adjacent restricted crosssection portion 3 of fuse link 1results in a magnetic blast removing the products of arcing formed uponfusion of link 1 through orifices 5 from points 3. if desired orifices 5may be sealed by a layer of tape 7 adapted to be'destroyed by the heatand pressure resulting from high fault currentarcs; Tape 7 permits abuild-up and sudden break-down of. pressure adjacent the points of thelink where. are initiation occurs, and such a consecutive build-up andbreak-down of pressure is conducive to more effective interruption ofelectric circuits. If de-. sired, a body of a pulverulent cooling mediumas, for instance, quartz sand may be arranged at the downstream ends oforifices 5. In the embodiment of the invention shown in Figs. 1 to 5,inclusive, such a body of cooling medium has been omitted but such abody of cooling medium has been shown in the embodiment of the inventionillustrated in Fig. 7. Link 1, insulating barrier 6- and hood 5 may bejoined together. by cement seals8 to form a self-sustained structuralunit, as clearly shown in Fig. 5. Cement seals 8' should be gas-tight topreclude any escape of products of arcing from the space within hood 4,except through orifices 5.

The portion of restricted cross-section or neck 3 of fuse link 1 isdesigned to have a current-limiting action, i. e. to fuse initially andto thus initiate interruption of the faulted circuitbefore the faultcurrent can reach the peak value of the available short-circuit currentof the system into which the fuse is inserted. The U-shaped portion 2,3, 2 of link 1 has its current-limiting neck 3 substantially at thecenter thereof, and orifices S are substantially in registry with neck3.

Referring now to Figs. 6 and 7, the current-carrying parts of this fusecomprise two spaced angle members 9 having a considerablecurrent-carrying ability and heat absorbing capacity. Members 9 arepreferably made of copper. Current bridge 10, preferably made of silver,establishes a current path from one of the angle members 9 to the other.Current bridge 10 is provided with a current-limiting portion of reducedcross-section or neck substantially identical with the current-limitingportion of reduced cross-section or neck shown in Fig. 1. Members orbars 9 have a considerably larger cross-sectional area thancurrent-bridge or link 10. As indicated in Fig. 7, the insulatingbarrier or spacer 6 is inserted into the gap formed by the two spacedangle members 9. Both the angle members 9 and the current bridge 10 arecovered by a first hood 4' having one or more venting orifices 5substantially in registry with the current-limiting neck portion ofcurrent bridge 10. The downstream end of venting orifice S is sealed bycellulose tape 7' which is being readily destroyed by are generated heatand pressure. The first hood 4 is inserted into a second hood 4" whichhas the same geometrical configuration as, but is considerably largerthan, the first hood 4'. The space between the first hood 4 and thesecond hood 4" is filled with a granular or pulverulent cooling medium11, preferably chemically pure quartz sand. A packing 12 keeps thecooling medium 11 within the U-shaped space formed between hoods 4' and4". The angle members or terminal connectors 9, the center insulator orspacer 6' and the two hoods 4 and 4" are bonded together by means ofcement joints 8 to form a self-sustained structural unit.

The composite structure shown in Fig. 6 is a sub-assembly unit which isbeing inserted as such into the hood 4. Current bridge 10 may bespot-Welded or brazed to the angle members or terminal connectors 9.Hood 4 may be provided with a pair of notches (not shown) permitting tomove the two aligned parts of angle member 9 into hood 4", as shown inFig. 7.

If the pulverulent cooling medium 11 is quartz sand, fusion thereof willoccur substantially simultaneously with the vaporization of currentbridge 10 by the heat of the interrupting arc. Fused quartz forms a bodyglasslike character, frequently referred to as fulgurite, which is agood conductor of electricity as long as it is at an elevatedtemperature, Since that body of glass-like character is formed outsideof hood 4, relatively remote from the arcing zone, the danger ofdestruction of the fuse by a post-interruption follow-current throughthe fulgurite, which often impairs the proper operation ofcurrentlimiting fuses, is virtually non-existent.

It will be understood that although but two embodiments of thisinvention have been shown and described in detail the invention is notlimited thereto and that the illustrated embodiments may be modified, orother embodiments made, without departing from the spirit or scope ofthe invention as set forth in the accompanying claims.

It is claimed:

1. A fuse comprising an insulating barrier, a fuse link having acurrent-limiting restricted cross-section portion situated between apair of relatively wide cross-section portions arranged at angles ofsubstantially 90 degrees with respect to said restricted cross-sectionportion, said restricted cross-section portion being substantially inregistry with one edge of said insulating barrier and said pair ofrelatively wide crosssection portions sandwiching said barrier, and aninsulating hood mounted on said link and having a venting port adjacentsaid restricted cross-section portion of said link.

2. A fuse comprising an insulating barrier, a fuse link having acurrent-limiting restricted cross-section portion situated between apair of relatively wide cross-section portions arranged at angles ofsubstantially degrees with respect to said restricted cross-sectionportion, said restricted cross-section portion being substantially inregistry with one edge of said insulating barrier and said relativelywide cross-section portions sandwiching said insulating barrier, atfirst insulating hood mounted on said link and having a venting portadjacent said restricted cross-section portion of said link, a secondinsulating hood mounted on said link in spaced relation from said firsthood, and a granular cooling medium sealed into the space between saidfirst hood and said second hood.

3. A fusible electric proetctive device comprising in combination a pairof spaced angular brackets, an insulating spacer between said pair ofbrackets, a fusible current-carrying bridge interconnecting said pair ofbrackets arranged edgewise with respect to said insulating spacer, saidcurrent-carrying bridge having a point of restricted cross-sectionalarea to predetermine the point of initial fusion and are initiation, anda substantially gas-tight enclosure for said current-carrying bridgehaving venting means only immediately adjacent said point of restrictedcross-sectional area of said current-carrying bridge.

4. A current-limiting fuse comprising a ribbon-type fuse link having arestricted cross-section portion adapted to rapidly fuse at theoccurrence of large fault currents, said link being bent substantiallyin the shape of a U and having a pair of spaced terminals, 21 pair ofterminal bars each having a considerably larger cross-sectional areathan said link and each conductivelyconnected to and supporting one ofsaid pair of terminals, and a hood of insulating material supported bysaid pair of terminal bars, said hood having a recess adapted toaccommodate the bent portion of said link, and said hood having ventingmeans in the apex portion thereof to permit the escape of products ofarcing formed upon fusion of said restricted cross-section portion.

5. A fuse comprising a pair of spaced terminal bars having a relativelylarge cross-sectional area, a ribbontype fuse link having a relativelysmall cross-sectional area supported by and conductively interconnectingsaid pair of bars, said link comprising a zone of restrictedcross-sectional area adapted to rapidly fuse at the occurrence of largefault currents, said pair of bars and said link jointly defining aloop-shaped current path producing a zone of intense magnetic fieldconcentration adjacent the apex portion thereof, a cover of heatresistant insulating material supported by said pair of bars and havinga recess accommodating said link, and venting means in said hood for theproducts of arcing resulting from the fusion of said link, said ventingmeans being substantially coextensive with said zone of restrictedcross-sectional area and with said zone of field concentration.

References Cited in the file of this patent UNITED STATES PATENTS961,308 Murray June 14, 1910 999,780 Harris Aug. 8, 1911 1,287,462Schweitzer et al Dec. 10, 1918 1,413,997 Schweitzer Apr. 25, 19221,562,984 Murray Nov. 21, 1925 2,143,037 Smith, Jr. Jan. 10, 19392,443,017 Arone et al. June 8, 1948 2,506,304 Ludwig May 2, 19502,665,348 Kozacka Jan. 5, 1954 FOREIGN PATENTS 596,492 France Aug. 10,1925

